Spontaneous Assembly of Graphite Auto-Kirigami

We show, for the first time, the formation of graphite auto-kirigami (AK) structures using in-situ SEM nanoindentation. The low energetic requirements for out-of-plane bending of 2D materials have enabled the formation of a range of nanostructures that are readily coupled with fracture processes. Specifically, graphene AK are three folded nanoribbons that form up to 5 μm in length due to graphene-graphene adhesion during spontaneous folding, peeling and tearing after nanoindentation. This process offers a route towards mechanical self-assembly of 2D heterostructures. The initial nanoribbon must first overcome the bending energy required to fold onto the adjacent 2D material; the formation rate should then decrease dramatically as the bending stiffness increases with the number of layers. However, we report graphite AK structures with 2, 3, or 4 leaflets that are 1-3 μm long and an order of magnitude thicker than graphene AK. They assemble spontaneously after nanoindentation of substrate-supported graphite and, as shown by in-situ SEM experiments, initiate via localized elastic buckling coupled to a fracture event beneath the tip. Using molecular dynamics simulations of few-layer graphene, we demonstrate an equivalent indentation-triggered film delamination process as we approach the 2D limit. Accordingly, we discuss how the initial graphene AK state is a limiting case of graphite AK for which membrane strain dominates over bending strain.